This invention relates to the treatment of highly concentrated formaldehyde in producing solid paraformaldehyde. The technology improves the solubility and storage life of paraformaldehyde. The product is used mainly in paint and coating industries.
Paraformaldehyde is a solid form of 80% or more formaldehyde. Typically the formaldehyde concentration ranges from 90% to 96%. Paraformaldehyde is thought to be poly(oxymethylene) glycol, HO--(CH.sub.2 O).sub.n --H, with n=8-100. Generally, it is manufactured by concentrating an aqueous hot formaldehyde solution under reduced pressure. Upon cooling, the resulting solution solidifies. Paraformaldehyde produced by this method is generally not stable over time. Immediately after it is produced, paraformaldehyde exhibits excellent solubility in water and organic solvents such as butanol. It dissolves readily in water or alcohol by hydrolysis or depolymerization to yield free formaldehyde. However, its solubility in water and solvents decreases with time or with storage at temperatures generally greater than about 35.degree. C. This change in solubility is presumably due to changes in the molecular weight of paraformaldehyde. To eliminate this phenomena, many stabilizers and inhibitors have been proposed. Until now, however, few have been successful in retarding the aging phenomena associated with paraformaldehyde. Generally, the degree of solubility depends largely on the degree of polymerization or the chain length of the polymer, n. So, it is desirable to control the polymerization of formaldehyde to paraformaldehyde. However, in most of the paraformaldehyde examined, not all of it dissolves in the solvent. That is, part of the paraformaldehyde remains as a solid and does not go into solution. These insolubles are believed to be poly(oxymethylene) glycol ethers. Hereafter referred to as ethers or insolubles. It is believed that these ethers affect physical properties, eg., solubility of paraformaldehyde. These ethers are formed by the reaction between paraformaldehyde and methanol in the presence of acid such as formic acid. As stated earlier, a desirable paraformaldehyde product would dissolve rapidly, produce little to no insolubles and would not continue to polymerize upon storage. It has been reported that during polymerization, formaldehyde reacts with alcohol, e.g., methanol, to form glycol ethers. This reaction is acid catalyzed. Typically formic acid exists in low concentration in the paraformaldehyde and serves as the catalyst for the reaction. These ethers are not soluble in water and hence remain as fine particulate matter upon dissolution of the paraformaldehyde. Therefore, the degree of polymerization, concentration of formic acid and methanol attribute directly to the insolubility and insolubles problems associated with paraformaldehyde. The most desirable paraformaldehyde would have a low degree of polymerization, high degree of solubility, little or no insoluble, and, most importantly, would not change with time or at storage temperatures greater than about 35.degree. C.
Paraformaldehyde is manufactured from hot concentrated formaldehyde solutions wherein the formaldehyde varies from about 30% to about 90%, most commonly about 80% formaldehyde. Various methods exist for manufacturing the solid paraformaldehyde: (1) solidify the concentrated formaldehyde solution in a reaction vessel, with or without catalyst and mechanically break up the mass formed; (2) pour the reaction contents on to a chilled surface, e.g. conveyor belt; (3) pour reaction contents over a heated roller device; (4) utilize a prilling tower whereby concentrated formaldehyde solution is fed into a tower cooled by current of air or inert gas. These methods produce either lumps, flakes, or spherical solids (also referred to as prills). EPO-716,104 A1. Conventionally, paraformaldehyde is manufactured by vacuum evaporation of aqueous formaldehyde. For example, Sumitomo Chemical Company concentrates formaldehyde solution to 80% by weight by fractional vacuum distillation. This process, however, allows for the rapid build up of formic acid in the resulting product, Sumitomo Chemical Company, British 869,764, Jun. 7, 1961.
Previously, Celanese Corporation patented a continuous two-stage vacuum evaporation of an aqueous formaldehyde solution with a pH of about 2.9 to about 3.5, U.S. Pat. No. 2,568,016, and U.S. Pat. No. 2,568,018. In the first stage, a 60% to 80% solution is heated continuously at 45 to 70.degree. C. under 25 to 100 mm Hg. During the second stage the formaldehyde solution is further concentrated to 80 to 90% by heating between 70 to 90.degree. C. at 100 to 200 mm Hg. The concentrated formaldehyde is maintained at 100 to 130.degree. C. for approximately 180 minutes. The resulting liquid is sent to a rotary flaker to produce solid chips of paraformaldehyde.
Literature has shown that the rate of formaldehyde polymerization can be controlled by the addition of catalysts. Both acids, U.S. Pat. No. 2,519,550, and German 1,112,505, and bases, U.S. Pat. No. 2,568,018 and U.S. Pat. No. 3,772,392, are said to accelerate the polymerization. Examples of the acidic catalysts are boric acid, sodium tetraborate, U.S. Pat. No. 2,519,981 and U.S. Pat. No. 2,519,550, and oxalic acid, Germanl 1,112,505.
In another effort to control the molecular weight of the paraformaldehyde, altering the drying conditions of concentrated formaldehyde has been proposed. A current of air containing either an acid or base in an amount sufficient to alter the pH of the paraformaldehyde is used in the drying process, U.S. Pat. No. 2,568,018. For instance, a treatment with triethylamine yields finely divided particles of paraformaldehyde with below 3% moisture. However, neither the molecular weight or the polymer chain length was reported to be lower.
All of these methods produce paraformaldehyde with a high molecular weight and hence, low solubility. Thereafter the paraformaldehyde becomes sticky, and difficult to flow or store. Methods are reported to make a product with high solubility, or low degree of polymerization; one is to add alcohol, generally at least 20%, such as methanol to the formaldehyde feed, DE 884,947 and U.S. Pat. No. 2,823,237; another is to add stabilizers, such as triazine, British 1,028,804, or an aliphatic amine, diamine, tertiary amine, or hydroxylamine and sufficient base to essentially neutralize the solution to the aqueous solution of formaldehyde, GB 931,892. The addition of amine has, to date, only been reported in the production of paraformaldehyde flakes or lumps; not in the production of prills which requires different technology to produce.
The polymerization process of formaldehyde is said to continue even after the paraformaldehyde flakes or prills are formed. Therefore the molecular weight of paraformaldehyde increases during storage. This causes the desired paraformaldehyde product to lose solubility, sometimes rapidly, upon storage. Polymerization inhibitors such as hydantoins, U.S. Pat. No. 2,519,981 and U.S. Pat. No. 2,519,550, and pentaerythritol, German 1,112,505, are reported to prevent this undesirable aging process. Other reported effective inhibitors include aliphatic and cyclic amines, and amino acids, French 1,486,060 and DE 701022. Hexamine in water and solvents (methanol) have been reported to inhibit polymerization of paraformaldehyde during storage, Japanese 73/48-17,250 and 73/48-8,603.
In spite of numerous reported methods to inhibit aging effects, the art indicates the continuing need to produce paraformaldehyde which even after storage has a low dissolution time and virtually no insolubles.